1. Field of the Invention
This invention relates generally to a double-cogged V-belt, more particularly to a double-cogged V-belt with aligned upper and lower cogs having a particular combination of cog profiles, and specifically such a V-belt adapted for a variable speed transmission.
2. Description of the Prior Art
The belt plays an important role in the operation of variable speed power transmission systems or drives, such as used in scooters, motorcycles, snowmobiles, all-terrain vehicles, cars and industrial applications. In one exemplary design of variable speed transmission (“VST”), the belt is a flexible element which connects two pairs of sheaves through friction to transmit power from the driving shaft to the driven shaft. Each pair of sheaves includes a fixed sheave and a movable sheave. By controlling the axial movement of movable sheaves, the speed and torque ratio may be changed. During operation, the belt sustains extreme longitudinal tension and bending and transverse compression. To achieve maximum performance, efficiency, and durability, one of the main challenges the belt design faces is meeting contradictory requirements, namely high longitudinal flexibility but high transverse stiffness while maintaining proper side contact. The general approach to this challenge has been to form alternating thick and thin sections on one or both sides of the belt, known as cogs or teeth, and roots, also known as valleys, grooves or notches, respectively. Cogs are intended to provide the thickness and stiffness needed for transverse stiffness, while the roots or notches are intended to provide the needed longitudinal bending flexibility. Cogs may be formed or applied on the inside or lower side of the belt, or cogs may be applied to the outside, i.e., the backside or upper side of the belt. Alternately, cogs may be applied to both the lower and upper side of the belt, resulting in a double-cogged belt.
The challenge for conventional V-belts for fixed sheaves or single-speed drives is similar, but not as severe. V-belts for VST generally need to be relatively wider and thinner than V-belts for fixed drives in order to accommodate a range of movement radially inward and outward in the variable sheaves. The resulting relatively wide aspect ratio of VST belts makes transverse stiffness more difficult to achieve, especially with the shifting movements placing increased transverse loads on the belt. On the other hand, since V-belts in fixed drives need not move in or out, the aspect ratio of the belt can be such that sufficient transverse stiffness is more easily achieved. Thus, while the use of cogs or notches is common to both fixed- and variable-speed V-belts, a fixed-speed V-belt cog design may not perform well in a VST.
Representative of the art is U.S. Pat. No. 4,276,039 which discloses a double-cogged V-belt for fixed drive with aligned upper and lower cogs. Such early designs have become disfavored because of perceived problems caused by alignment of the cogs and particularly alignment of the roots. Alignment of the upper and lower roots creates relatively thin web sections between the cogs where bending stresses may be highly concentrated and where bending radii can become very small. This results in cracking of the belt body in the root areas, cord fatigue and early failure. U.S. Pat. No. 4,276,039 applies a canvas cover over both lower and upper belt surfaces to help prevent cracking.
Also representative of the art is U.S. Pat. No. 4,708,703, which discloses a V-belt for a VST with aligned upper and lower teeth and grooves. The teeth are preferably covered at their tops with a reinforcing and stiffening element to deal with the problem of buckling.
The art contains many attempts to optimize the profile, including the shape, pitch, depth, alignment, and the like, of the upper and lower cogs of double-cogged V-belts. U.S. Pat. No. 6,620,068 discloses a raw-edge double-cogged V-belt for variable speed drives having curvilinear cogs on the inside and outside. The number of outside cogs are twice the number of and aligned with the inside cogs. JP 2002-089631A discloses a dual cog V-belt with more upper cogs than lower cogs, but less than twice as many so that the alignment or phase of the upper and lower cogs is variable.
A number of patents teach that the upper and lower cogs should be staggered, i.e., exactly 180° out of phase and of equal pitch or number. U.S. Pat. No. 1,890,080 discloses staggered rounded cogs of equal size and shape. U.S. Pat. No. 2,699,685 discloses staggered blocky-shaped cogs of equal size and shape with the grooves of one section vertically opposite the cogs of the other section in order to avoid weak spots and so that the thickness of the belt is the same all over.
JP 2002-031192A discloses a variation on a staggered double-cogged V-belt for VST applications wherein equal-numbered upper and lower cogs are not exactly in phase or out of phase, but phase shifted an amount somewhere in between, preferably from a tenth to half of the pitch. That publication teaches that lower and upper cog parts should not align or correspond so the belt thickness does not get extremely small thus preventing stress concentration and early crack initiation in that region. Finite element method (“FEM”) analysis was apparently used to design an improved phase-shifted staggered profile and to confirm this effect. Increased phase shift up to half a pitch resulted in reduced root cracking.
In designs such as disclosed in JP 2002-031192A and JP 2002-089631A in which there are more upper cogs than lower cogs, the alignment of the upper and lower cogs is variable. In such a design, unequal pitches results in a “weak link” at the position around the belt where the upper and lower roots are most closely aligned. Root cracking may be observed to begin at this aligned root position. Even so, this design seems to be the most optimized design in the current market for double-cogged variable-speed V-belts.
Reference is made to co-pending U.S. patent application Ser. No. 12/217,026 filed Jul. 1, 2008, the contents of which are incorporated herein by reference in their entirety.